This invention relates to a magnetic objective lens for use in a scanning electron microscope in which the working distance is largely varied.
In a conventional scanning electron microscope, a magnetic objective lens shown in FIG. 1 is incorporated. In the figure, the objective lens 1 comprises an excitation coil 2 wound around a lens axis 3, yokes 4 and 5 housing said excitation coil 2, an upper magnetic pole piece 6 attached to the annular end of the yoke 4, and a lower magnetic pole piece 7 attached to the annular end of the yoke 5. The magnetic field, which is symmetrical with respect to the axis 3, is formed between the upper and lower magnetic pole pieces 6 and 7 for converging the electron beam 8 from an electron gun (not shown) along the axis 3 on the surface of a specimen 9. The bore diameter D1 of the lower magnetic pole piece 7 is smaller than the bore diameter D2 of the upper magnetic pole piece 6, so that the strong magnetic field of the objective lens does not extend to the specimen. Position of the specimen 9 on a specimen stage 10 is varied by handling the specimen stage 10. The working distance W.D. (i.e., distance between the lower surface of the lower magnetic pole piece 7 and the specimen surface) is also varied by handling the specimen stage 10.
In ordinary observation using the scanning electron microscope, the W.D. is set at a comparatively long distance because of the following reasons.
(i) Deep "depth of focus length" due to long focal length of the objective lens is obtained, so that the scanning image of the uneven surface of a bulk specimen is observed clearly.
(ii) High takoff angle .theta. in the detection of the X-ray 11 radiating from the specimen 9 is obtained under condition of long W.D., so that the accurate X-ray measurement is possible.
(iii) Observation of a ferromagnetic specimen which should not be subject to a strong magnetic field becomes possible, because the lens magnetic field extended at the specimen positioned at long W.D. is quite weak.
On the other hand, in order to obtain a high resolution image, the diameter of the electron beam irradiating the specimen must be maximally reduced through the medium of the incorporated magnetic lens system. In conventional scanning electron microscopes, the main factor which hinders the reduction of the electron beam diameter is theoretically and practically a spherical aberration of the objective lens. Further, a chromatic aberration of the objective lens becomes a non-negligible factor as compared with said spherical aberration in a special observation case (particularly in the condition of the very short focal length of the objective lens). Quantity of said spherical aberration is proportional to Cs.multidot..alpha..sup.6 where Cs is a spherical aberration coefficient and .alpha. is the semi-angle of the electron beam subtended by the objective lens at the converged spot on the specimen. An amount of chromatic aberration is proportional to Cc.multidot..alpha..sup.2 where Cc is a chromatic aberration coefficient.
From the above, in order to reduce the diameter of the electron beam and thereby increase the resolution of the image, it is necessary to provide an objective lens having a small spherical aberration coefficient. And it is essential to shorten the objective lens focal length which is roughly proportional to the spherical and chromatic aberration coefficients. In other words, to minimize the spherical aberration in the objective lens, the W.D. corresponding to the focal length must become extremely short. In observation under the condition of short W.D., however, the above advantages (i), (ii) and (iii) are not ensured, and the value of the spherical aberration coefficient is not yet adequately reduced.
In order to obtain a higher resolution scanning image, there has been proposed an apparatus in which a specimen is placed in the gap between the magnetic pole pieces, as shown in the broken line 9', and the objective lens is strongly excited for the short focal length so as to attain a small spherical aberration coefficient. The proposed apparatus is, however, disadvantageous in that since the specimen is confined in a small space, a limitation is placed on the size of the specimen to be inserted through a side hole (not shown) of the lower pole piece, and it is difficult to effect desirable position control (for shift, rotation and inclination) for the specimen.